1. Field
The present invention is concerned with switching the stroke length of one or more reciprocating pistons of machines including reciprocating gas compressors and vacuum or other pumps and other devices including scotch yoke compressors such as shown in U.S. Pat. No. 4,838,769, in which the reciprocating motion of the pistons is effected by the orbiting of crankpins which are attached to said pistons by connecting rods or other connecting structures having bearings rotatably mounted on said crankpins.
In particular, the invention concerns gas compressors, especially multi-cylinder refrigerant compressors, in which the connecting rod bearing of at least one piston is mounted on an eccentric cam rotatably mounted on the crankpin. This cam is angularly adjustable by reversal of the crankshaft drive motor and thus the crankshaft rotation to switch to either a lengthened or shortened crankpin throw and piston stroke, depending, by design, on the direction of rotation of the crankshaft. Such stroke or throw switching can be engineered to give desired high pressure refrigerant output capacities such that the compressor efficiency can be maintained more easily under markedly varying load requirements.
Another and preferred aspect of the invention concerns a unique electrical circuitry for operating the crankshaft drive motor, whereby reversal of the motor for reducing or eliminating the throw takes the motor off the normal run winding and places it on a more efficient winding of reduced current capacity, in particular, the start winding.
2. Prior Art
Throw switching structures for which the present invention finds particular application are shown and described in U.S. Pat. Nos. 4,479,419; 4,236,874; 4,494,447; 4,245,966; and 4,248,053, the disclosures of which with respect to general compressor construction and also with respect to particular structures of cylinder, piston, crankshaft, crankpin and throw shifting mechanisms are hereby incorporated herein by reference in their entirety. With respect to these patents the crankpin journal is complex and comprised of an inner and one or more outer eccentrically configured journals, said inner journal being the outer face of the crankpin shaft, and the outer journal(s) being termed “eccentric cams or rings” in these patents, and being rotatably mounted or stacked on said inner journal. The bearing of the connecting rod is rotatably mounted on the outer face of the outermost journal.
In these patents, as in the present invention, all journal and bearing surfaces of the power transmission train of the shiftable throw piston from the crankshaft to the connecting rod are conventionally circular and allow structurally unhindered rotative motion, within design limits, of the outer journal(s) on the inner journal and of the connecting rod bearing on the outermost journal. This rotative motion, in either direction, will, thru the eccentricity of the outer journal surface of the outermost journal relative to its inner bearing surface, shift the radial distance of the orbital axis of the crankpin from the axis of rotation of the crankshaft and thus change the throw of the crankpin and the stroke of the piston. The present invention will be described below in reference to a single cam mounted on a crankpin shaft.
As described in, e.g., said U.S. Pat. No. 4,479,419 and with reference to the structure numbering therein, the angular positioning of the cam 38 on the crankpin 34 is accomplished by providing a pair of drive stops (not numbered in the said 4,479,419 patent, but numbered 58, 60 in the 4,494,447 patent as “end points”) which are angularly spaced on a portion of the crankshaft such as the crankpin 34, and a driven dog 48 provided on the cam 38. These stops and the dog are angularly positioned with respect to each other such that upon rotation of the crankshaft in one direction one of the stops will first engage one side of the dog and rotate cam 38 to a first prescribed angular position on the crankpin to produce one piston stroke length. Conversely, reversing the rotation of the crankshaft will terminate this first engagement and cause the other of the stops to rotate to and engage an opposite side of the dog and rotate the cam to a second prescribed angular position on the crankpin to produce another piston stroke length. These angular positions are alternatively characterized herein as “end point(s)” or “dog-stop” junction(s) or “contact junction(s)”, all hereinafter termed “junction(s)”.
It is noted that at least a portion of the rotation of the cam relative to the crankpin to either of its endpoints can also result from the inertia of the cam or the rotational drag of the strap end bearing of the connecting rod acting on the outer journal surface of the cam.
It is apparent that for a given fixed crankpin throw the maximum possible magnitude of the piston stroke shift will depend on the degree of eccentricity between inner bearing surface and the outer journal surface of the cam. A larger eccentricity will allow an increased or reduced throw depending on the angular position of the cam on the crankpin. Therefore, a properly configured eccentricity will allow the said orbital axis of the crankpin to become coincident with the axis of the crankshaft, thus bringing the crankpin throw and the piston stroke to zero, and thus pacifying the throw, piston and cylinder. It is noted that in this zero stroke or passive mode, the completely pacified piston will remain, theoretically, one half way between its normal top dead center and normal bottom dead center positions during further operation of the compressor in the reduced capacity mode.
It is to be particularly noted, that as mentioned above, all of the journals and bearings involved in this power transmission train are essentially perfectly circular within, of course, modern machining capability, and their rotational contacts with one another are practically frictionless. Thus arises the conundrum that if only one side of the dog is in engagement with a stop at any given time, what is to prevent disengagement of the junction and the consequent rotation of the cam on the crankpin during periods when the cam is being driven by the stop with only minimal force? Such a disengagement would produce a plethora of unplanned piston strokes, which of course would significantly thwart the effort to maintain maximum compressor efficiency under varying load requirements. The apparent answer is that the junction can be maintained simply by the inertia of the cam during such periods.
Applicants extensive experimentation however, with such complex crankpin journals under typical compressor operating conditions has confirmed that such cam inertia is essentially ineffective to prevent disengagement, i.e., instability of the junction and the throw shift under the dynamic forces present, even in a theoretically completely pacified cylinder.
Taking, for example, the particular case wherein the cam has been rotated on the crankpin to produce a zero piston stroke, any significant “incidental” pressure above ambient, i.e. above the low pressure side of the compressor, developed in the pacified compression chamber by, e.g., high pressure gas leakage across the discharge valve, will act on the piston and connecting rod and cause the cam to rotate on the crankpin at least within the first quadrant (see FIG. 3) and disengage the junction. Such disengagement can range rapidly from barely detectable to several degrees and result in the generation of unwanted piston strokes of various lengths. Also, a further and quite severe problem resulting from such disengagement which our investigations have revealed is that of the generation of considerable metallic clacking or clicking noise generated by the rapid and forceful reengagement of the stop and dog, particularly in the second quadrant, as the disengagement angle declines to zero with a drop in said incidental pressure towards or even below ambient.
Further, taking the case where a previously completely pacified piston has become fully stroked by reversal of the crankshaft rotation, incidental pressure applied to the piston on the inception of its suction stroke by way of compressed gas which was not discharged on the compression stroke, i.e., reexpansion gas, will tend to cause an essentially immediate disengagement of the junction in the first quadrant. This disengagement can then be continued and perhaps amplified in the second quadrant as the linear velocity of the crankpin declines for the bottom dead center turn while the linear inertia of the combined masses of the piston and connecting rod maintains a rotating force on the cam and tends to advance it and the dog further beyond the stop. Under such conditions, the eventual reengagement of the stop and dog in a late section of the second quadrant is with substantial momentum and impact.
Still another major contributor to junction instability is that immediately upon completion of the bottom dead center turn of the crankpin, the diminished pressure existing in the compression chamber will allow the ambient pressure rapidly force the piston further into the cylinder and thereby rotate the cam in advance of the stop and cause a substantial degree of disengagement. Then, as the pressure in the compression chamber rapidly increases on the compression stroke, the linear velocity of the piston rapidly declines relative to the orbital velocity of the crankpin and the stop catches up with and impacts against the dog, with considerable force and noise.
Referring to the aforementioned patents, only the 4,494,447 patent even alludes to any destabilizing phenomena, and then only with a glancing mention in column one that gas thrust, piston rod inertia, and centrifugal and gas torque reversal forces contribute to cam instability. In what context and in what relationship however, to, e.g., a zero stroke piston mode is not readily apparent from this patent. Also, in column one of that patent it is stated that “—forces which generally tend to prevent the possibility of oscillation are friction forces, various drag loads, and cam inertia forces.” This statement appears to be a recitation of those forces which are inherently present, in varying degrees, in all refrigeration compressors, and sheds no light on a solution to the instability problem, particularly with respect to a zero piston stroke mode.
This patent then goes on to disclose an actual stabilizing structure constituting its invention, which structure is characterized as aiding in holding the cam in the desired position. This structure comprises end stops 58, 60 which are preferably spaced about 270° apart such that a substantial centrifugal force torque “CFT” will develop tending to maintain the stops and dog in contact at the endpoints of the cam rotation, as shown in FIGS. 4 and 5 of the patent.
Also as disclosed in this patent, this CFT can be generated by repositioning the center of mass 62 of the cam away from the throw axis which passes thru the crankshaft axis 30a and the crankpin axis 32a, as shown in FIGS. 6 and 7 of the patent. It is particularly noted that the stabilizing structure of this patent is not intended to, nor can it function to provide a zero stroke piston mode. Further, it has been Applicants' experience that even where a reduced stroke is to be provided, a CFT solution is inadequate to prevent junction destabilization and unacceptable noise in conventional compressor operations. Obviously, as used herein, the CFT generated by this patent and by the present structure is a CFT which tends to make, rather than break the junction(s).